One of the first characteristics of a wave is that they can transport energy and information from one place to another through a medium, but the medium itself is not transported. A disturbance, or change in some physical quantity, is passed along from point to point as the wave moves. In the case of light waves or radio waves, the disturbance is a changing electric and magnetic field; in a sound wave, it is a change in pressure and density. In both cases, the medium reverts to its undisturbed state after the wave passes.
Two types of waves will be discussed in this section; transverse waves and compressional waves. A transverse wave is one in which the vibrations are at right angles to the direction the wave is traveling (ex. Waves on a rope). A compressional wave is one in which the vibration is in the same direction as the wave is traveling (ex. Sound waves in the air).
Have you ever seen stadium waves created by excited fans at a baseball game? Stadium waves, water waves, microwaves, sound waves and radio waves all have one thing in common. They all transfer energy from one place to another.
Water waves are probably the easiest type of wave to visualize. If you have been in a boat, you know that approaching waves bump against the boat but do not carry the boat along with them. The boat just moves up and down as the waves pass by. Like the boat, the water molecules on the surface of the lake move up and down, but not forward. Only energy carried by the waves moves forward.
Waves are rhythmic disturbances that carry energy through matter or space.
Water waves transfer energy through the water. Earthquakes transfer energy in powerful shock waves that travel through Earth. Both types of waves travel through a medium. A medium is a material through which a wave can transfer energy. This medium may be a solid, a liquid, a gas or a combination of these. Radio waves and light waves, however, are types of waves that can travel without a medium.
Two types of wave motion can carry energy. Figure 1 shows how you can make a transverse wave by snapping the ends of a rope up and down while a friend holds one end. Figure 2 shows how a compressional wave should look.
Notice that as the wave moves, some of the coils are squeezed together just as you squeezed the ones on the end of the spring. The crowded area is called compression.
The compressed area then expands, spreading the coils apart creating a less dense area. This less dense area of the wave is called a rarefaction. Does the whole spring move? Tie a piece of string on one end of the coils and observe the motion. The string moves back and forth with the coils. Therefore, the matter in the medium does not move forward with the wave. Instead, the wave carries only the energy forward.
Transverse waves have wavelengths, frequencies, amplitudes and velocities. Compressional waves also have these characteristics. A wavelength in a compressional wave is made of one compressional, and one rarefaction as shown in Figure 3. Notice that one wavelength is the distance between two compressions or two rarefactions of the same wave. The frequency is the number of compressions that pass a place each second. If you repeatedly squeeze and release the end of the spring three times each second, you will produce a wave with a frequency of 3 Hz.
The high, almost dense points of a wave are called crests; the lowest points are called troughs. Waves are measured by their wavelength. Wavelength is the distance between a point on one wave and the identical point on the next wave, such as from crest to crest or trough to trough.
Frequency and Pitch
Frequency is the number of waves that pass through a point in one second, expressed in Hertz (Hz). Pitch is the highness or lowness of a sound. The pitch you hear depends on the frequency of the sound waves. The higher the frequency, the higher the pitch; the lower the frequency, the lower the pitch. A healthy human ear can hear sound frequencies from about 20 Hz to 20,000Hz. As people age, they often have trouble hearing high frequencies.
Do high-pitched sounds travel at a different speed than low-pitched sounds? Let’s ask this question differently. If you were at an outdoor band concert (without electronic amplification) and the conductor gave the downbeat to the band, would the sound of the piccolo get to you before or after the sound of the tuba?1
Your experience will help to tell you that if there were much of a difference in the arrival times between high and low pitches, not only would it be difficult to keep the performance together, but also it would sound quite different up close to the band than further away. In fact, sound in the normal audible range travels at a constant speed independent of pitch.
Most people cannot hear sound frequencies above 20,000 Hz The frequency of the human voice range that carries information extends from about 250 to about 2000 Hz in a normal conversation. Bats, however, can detect frequencies as high as 100,000 Hz. Ultrasound waves are used in sonar as well as in medical diagnosis and treatment. Sonar, or sound navigation ranging, is a method of using sound waves to estimate the distance to, size, shape and depth of underwater objects.
Sound must have a medium (liquid, gas or solid) through which to travel. It cannot travel through a vacuum. A vacuum is a space that is empty of everything, even air. If you put a ringing alarm clock into a jar and pump the air out of the jar, the sound of the ringing will decrease as you pump out the air. When most of the air molecules are out of the jar, not enough molecules remain to form sound waves and the ringing sound stops.
Imagine the sound you hear when a fire truck with its siren on rapidly approaches and then passes you. As the truck is moving toward you, the pitch of the siren sounds higher. The motion of the siren toward you compresses the sound waves closer together. This increases the frequency of the sound waves striking your ear. As a result, the pitch you hear is higher. As the siren moves away the waves are pulled farther apart. This decreases the frequency and you hear a lower pitch. This change in wave frequency is called the Doppler effect. The Doppler effect is observed when the source of sound is moving or when the observer is moving.
Water waves can be described by how high they appear above the normal water level. Amplitude describes wave height. Amplitude is the distance from the crest (or trough) of a wave to the rest position of the medium. See Figure 4. Amplitude relates to the amount of energy carried by the wave. Waves that carry great amounts of energy have large heights or amplitude; waves that carry less energy have smaller amplitudes.
When two waves of the same frequency reach the same point, they may interfere constructively or destructively. If their amplitudes are both equal to A, the resultant amplitude may be anything from zero up to 2 A. The same is true of a wave that reflects back on itself after hitting a hard surface.2
The frequency of a wave is the number of wave crests that pass one place each second. Frequency is measured in hertz (Hz). One hertz is the same as one wave per second. To increase the frequency of the wave in Figure 1, you move the rope up and down faster. As the frequency increases, the wavelength decreases.
Sometimes you may want to know how fast a wave is traveling. For example, earthquakes below the ocean can produce giant tidal waves. You would want to know how soon a tidal wave would reach you, if you needed to seek shelter. Wave velocity, (v) describes how fast the wave crests move.
Wave velocity can be determined by multiplying the wavelength and frequency. Wavelength is represented by the Greek letter lambda, ». If you know any two variables in an equation you can find the unknown variable. Velocity = wavelength x frequency.
For sound waves, the sound velocity does not change with frequency for a given medium.